High-efficiency (&gt;50%, for example) RF or microwave power amplifiers are typically designed to operate in Class-B, Class-C, Class-E or Class-F modes. A common characteristic of these modes of operation is that in the absence of an input microwave signal, the solid state active transistor of the power stage(s) is biased at or near zero quiescent current. As the amplifier is driven by external microwave power, the operating current of the transistor rises to the designed level, thereby amplifying the microwave signal. FIG. 1 shows the typical variation of the operating current of a bipolar transistor biased for a high efficiency mode of operation.
A problem with such modes of operation is that the input junction of the device can be driven into reverse (avalanche) breakdown under large signal conditions. When a charge carrier travels in a semiconductor medium, it travels, on average, a "mean free path" before interacting with an atom in the lattice and losing its energy. In an npn bipolar transistor, as the base-emitter bias is increased to a large negative value, the electric field in the junction becomes large. Consequently, a charge carrier can gain sufficient energy from the field while traveling within its mean free path to free a bound electron upon impacting the lattice. The initial carrier and the free hole and electron created by the collision are then free to leave the region of the collision. This process is called impact ionization. When the base-emitter bias reaches a critical negative value, -V.sub.BE(BD) (the base-emitter junction breakdown voltage), the events of impact ionization are so numerous that the base current due to the ionized carrier becomes very large and the transistor ceases to function properly. A transistor in this condition is said to be in avalanche breakdown. The avalanche breakdown of semiconductor-semiconductor, metal-semiconductor, or metal-insulator-semiconductor junctions generates an excessive amount of white noise. An amplifier suffering from such noise is unacceptable for applications such as the transmitters and receivers commonly used in telecommunications devices. Evidence of the level of this noise can be taken from the fact that avalanche diodes are used as noise sources in instrumentation.